It is well known that the properties of silver halide dispersions (so-called emulsions) are strongly influenced by the environmental conditions in which the silver halide grains are formed. The influential parameters are numerous. They include the mixing rates of solutions of reactive silver and halide salts, the relative amounts of the reactants in the reaction mixture, the pAg, the pH and the temperature of such mixture, and the mechanical forces to which the mixture is subjected. In the art of silver halide emulsion preparation, numerous measures are known for influencing emulsion properties and, in particular, for improving the uniformity of the emulsion and the reproducibility of results under industrial manufacturing conditions.
The formation technique for silver halide emulsions wherein nucleation and growth all take place in a single container is commonly practiced. This technique has the disadvantage that the previously nucleated grains are recirculated past the mixer where additional silver salt and halide salt solutions are being added. This brings together previously nucleated grains into the same system as newly nucleating grains. The material in the container has a continuously changing content of the salt byproducts of the nucleating process, as well as a continuously changing competition for silver and halide with previously nucleated grains. Therefore, given these changing conditions, it is not surprising that the nucleated grains formed at different times are different.
It is known to prepare silver halide emulsions batchwise by introducing silver salt and halide salt solutions into a circulating volume of peptizer solution which is repeatedly recycled. Note U.S. Pat. No. 4,334,012--Mignot where recirculation and ultrafiltration is discussed. This procedure has the advantage over non-recirculating preparation systems that small amounts of peptizer can be used. The recycling of peptizer solution and the addition of reactants thereto can continue until a silver halide emulsion with a predetermined concentration of silver halide is attained.
U.S. Pat. No. 4,171,224--Verhille et al discloses a method of bringing together separate streams of silver nitrate and a halide salt in the reaction zone. Material is recirculated through the reaction zone and further subjected to a continuous flow mixing passageway that will provide backflow of material.
U.S. Pat. No. 4,775,617--Goda is directed to the production of monodisperse grains. However, the grain sizes are adjusted after nucleation and while the sizes of the final grains may be somewhat monodisperse the formation history of the grains are different and therefore their properties are not uniform. There is a need for the method of nucleation that would provide each silver halide crystal or grain with a uniform history. By this it is meant that each grain would have been subjected to the same concentration of halide ion and silver ion during formation. Further, the individual grains would each be surrounded by silver grains that were identical to the silver grains surrounding other of the final crystals. It is known that in nucleation a multitude of crystals are formed whereas during ripening and growth most crystals are dissolved and redeposited on the surviving crystals. The composition and size of the crystals which do not survive but are dissolved and deposited on surviving crystals also determine the history of the particles in a photosensitive emulsion.
In U.S. Pat. No. 4,879,208--Urabe, mixing in a separate mixer vessel to nucleate prior to transfer to a larger reactor vessel is disclosed as producing more uniform silver halide grains.
There is a need for a method of achieving uniform nucleation conditions such that all reacting elements have the same history of mixing and reaction with the nucleating stream. Further, there is a need for isolation of forming silver halide nuclei from already formed crystals so that formation of younger nuclei will not be influenced by the presence of older crystals.